Continuous wave

[2] Information is carried in the varying duration of the on and off periods of the signal, for example by Morse code in early radio.

Very early radio transmitters used a spark gap to produce radio-frequency oscillations in the transmitting antenna.

The signals produced by these spark-gap transmitters consisted of strings of brief pulses of sinusoidal radio frequency oscillations which died out rapidly to zero, called damped waves.

Manufacturers produced spark transmitters which generated long "ringing" waves with minimal damping.

Continuous waves could not be produced with an electric spark, but were achieved with the vacuum tube electronic oscillator, invented around 1913 by Edwin Armstrong and Alexander Meissner.

After World War I, transmitters capable of producing continuous wave, the Alexanderson alternator and vacuum tube oscillators, became widely available.

[3] The spurious noise emitted by a transmitter which abruptly switches a carrier on and off is called key clicks.

Continuous-wave radio was called radiotelegraphy because like the telegraph, it worked by means of a simple switch to transmit Morse code.

Non-directional beacons (NDB) and VHF omnidirectional radio range (VOR) used in air navigation use Morse to transmit their identifier.

Morse code is all but extinct outside the amateur service, so in non-amateur contexts the term CW usually refers to a continuous-wave radar system, as opposed to one transmitting short pulses.

Some monostatic (single antenna) CW radars transmit and receive a single (non-swept) frequency, often using the transmitted signal as the local oscillator for the return; examples include police speed radars and microwave-type motion detectors and automatic door openers.

[5] Optical communication in turn provided the hardware basis for internet technology, laying the foundations for the Digital Revolution and Information Age.